In transport means such as automobiles, railroad vehicles and airplanes, resistance spot welding or rivet connection has been employed to couple metal materials to each other. However, in recent years, attention has been given to friction stir spot welding as disclosed in Patent Document 1 or 2. According to friction stir spot welding, the metal materials are welded to each other by friction heat with a cylindrical rotating tool (welding tool) having a pin member at its front end. The rotating tool is configured to advanceable and retractable with respect to an object to be welded, and advances at a pressure or speed in a predetermined range while rotating at high speed to be pushed into (press-fitted into) the object to be welded (metal materials). The metal materials are softened at a site where the rotating tool is press-fitted, and the softened metal materials are stirred to weld the object to be welded.
The friction stir spot welding disclosed in Patent Document 1 uses only the pin member as the rotating tool and thus, for convenience of description, will be referred to as single-acting friction stir spot welding. On the other hand, the friction stir spot welding disclosed in Patent Document 2 uses a substantially cylindrical pin member and a substantially tubular shoulder member having a hollow for inserting the pin member thereinto as the rotating tool, and the pin member and the shoulder member can independently rotate, and advance and retract. For convenience of description, the friction stir spot welding of such configuration will be referred to as double-acting friction stir spot welding (refill friction spot joining). According to the double-acting friction stir spot welding, by adjusting timings of advancement and retraction of the pin member and the shoulder member, a recess formed by press-fitting of the pin member can be backfilled.